Could you, please, clarify why? Simply because of the fact that it is out of your responsibility to stylize any user content? Or, maybe, is there any additional reason (-s) like performance? I mean, does it cost to reset CSS?

Using a reset stylesheet was a defect that's been carried along since the origin of Ext JS.

Removing the reset stylesheet is not related to performance.

In general, you should be able to drop extjs.css into an existing page, and it should not change any of the existing style of the page. Now that the reset stylesheet has been removed, extjs.css shouldn't mess with any of the user created styles.

This can cause a pretty serious breaking change for many applications, and unfortunately happened during a minor version release, but overall it is a good thing.